In electric measurement technology, but in general also in physical measurement technology, a problem often occurs, in connection with a measuring arrangement consisting of a serial arrangement of a signal source, a measurement path with or without a transmission test object, and a detector, in measuring the electrical detector output signal in relation to its parameters (single frequency signals: amplitude, phase; random signals: output, correlation), and in part in relation to a reference signal (required during phase and correlation determination), in order in this way to draw conclusions as to the signal transmission within the measurement path (transmission) and thus the properties of the test object, or simply, if there is no test object, as to the properties of the source signal (emission).
Such measuring arrangements can be used for the examination of any type of physical transmission or emission process, including mechanical or thermal, but in particular acoustical, electrical, or optical processes, the detector being designed to change the physical parameter examined, e.g. pressure, field strength, etc., into an electrical voltage.
Noise signals having various origins may be superimposed on the signal representing the parameter to be detected. Certain noise signals may be connected with the examined transmission or emission process and are superimposed at the detector input on the component of the signal to be examined (e.g., optical process: incidence of scattered light; acoustical process: background noise). Others arise from noise sources of a nature not related to the process and falsify the detector output signal (e.g., noise in acoustic detectors--microphones--because of mechanical shock) as well because of further electrical noises which either are created in the detector (internal noise, interference direct current) or are superimposed on the detector output signal from outside noise sources (net ripple voltage, pick-up). Frequently, the presence of such noise signals renders the examination of weak desired signals in this simple arrangement impossible, because the desired component of the detector output signal often lies below the noise level.
It is known that if the signal transmitted in the measurement path which is to be measured is modulated with a periodic signal (identification modulation) prior to entering the detector, a separation of desired and noise components in the detector output signal becomes possible. This can be accomplished either by a direct modulation of the source signal or by insertion of a modulator element into the measurement path. Noise voltages can thus be filtered out by a narrow band evaluation (band pass filtering or synchronous detection) of the detector output signal on the modulation frequency.
Measuring systems are known in the field of microwave gas spectroscopy which modulate the absorption behavior of the gas sample by means of, for example, a low frequency electrical alternating field having a high voltage (strong modulation) and which evaluate the low alternating signals of the detector output signal according to the modulation frequency (Reinhard Reinschlussel: Ein Gassoektrometer hoher Nachweisempfindlichkeit im Millimeterwellenbereich [Gas Spectrometer with High Detection Sensitivity in the Millimeter Wave Range]; Dissertation, Bochum 1985).
In the field of correlation measurement technology of noise signals, it is known to advantageously provide the input signal of the correlator with a periodic 0.degree./180.degree. phase modulation in order to make possible suppression of the interfering direct current components of the correlator output signal and perhaps of additional noise voltages by means of filtering (e.g., see p. 17, FIG. 2.5 in Manfred Spaude: Ein impedanzunabhangiges Messverfahren zur Bestimmung von Zweipol-Rauschtemperaturen im Hochfrequenzbereich [A Measuring Method Independent of Impedance for the Determination of Two-pole Element Noise Temperatures in the High Frequency Range]; Dissertation, Bochum 1984).
Measurement of low electrical direct voltages or alternating voltages of low frequency is customarily performed by means of so-called "chopper amplifiers", which eliminate the drift problems of direct current-coupled amplifiers by "on-off" modulation and alternating current amplification.
Methods for the determination of the transmission damping, or loss, of a test object in a measurement arrangement consisting of a signal source, test object, detector, band pass filter and display are known from the field of electrical two-part parameter measurement technology, and achieve a direct identification modulation of the source signal (e.g., see p. I 12, FIG. 7 in H. Dalichau: Hochfrequenzmesstechnik [High Frequency Measuring Technology) in Meinke-Gundlach, "Taschenbuch der Hochfrequenzmesstechnik" [High Frequency Technology Handbook], Vol 1, publ. by K. Lange and K.-H. Locherer, Heidelberg: Springer 1986).
To determine the complex transmission function of the test object, the detector may also be in the form of a null detector which is supplied with a component of the identification modulated source signal which can be adjusted for amplitude and phase (parallel substitution method).
Further known transmission measuring methods (homodyne measuring methods) effect an identification modulation by means of a modulating element located ahead of or behind the measured object in the measurement path. A synchronous detector (balanced mixer) supplied with ann unmodulated component of the source signal is used for detection (e.g., see p. 5, FIG. 1.1, R. J. King: Microwave Homodyne Systems, Peter Peregrinus Ltd., Southgate House, Stevenage, England 1978).
The principle of identification modulation is also used in connection with automatic homodyne two-part measurement methods which use as a detector a single sideband detector (p. 133, FIG. 4.9, Burkhard Schiek: Me.beta.-systeme der Hochfrequenztechnik (Measurement systems for radio frequency technology) Heidelberg: Huthig, 1984) or an amplitude/phase detector with cascaded binary phase modulation elements. (Burkhard Schiek, Uwe Gartner, German patent application P34 25 961.3-35 of the Bargwargsuetband GmbH, Essen, W.-Germany).
It has proven disadvantageous in connection with use of all such types of identification modulation that crosstalk voltages appear at the detector output synchronously with the modulation signal, which crosstalk voltages are looped in via the electronic modulation components and cannot be filtered out. Because of the partially very high energy of the modulation signals (in particular in gas spectroscopy), they cannot be totally eliminated by shielding and limit the detection sensitivity of all devices.
As is also known (see King, supra., p. 38 et seq.), additional crosstalk signals occur in the use of identification modulation in particular in the field of homodyne two-part measurement methods. These signals are created by the reflection of components of the measuring signal at the not ideally matched modulator and lead to a signal from the measurement apparatus even when the test object is removed from the measuring arrangement (corresponding to infinitely high transmission loss) and thus falsely indicate a finite transmission loss of the test object. Even with test objects with finite loss these noise signals have a falsifying effect on the measurement results, if the amplitude of the signal transmitted by the test object is the same as or less than the amplitude as the noise signal.
It is known to reduce these reflections by the installation of directional elements or isolators in the measurement branch (see King, supra., pp. 38 et seq.). However, directional isolators are severely restricted in their desired frequency bandwidth because of their nature of non-reciprocal components and thus can only be used in a limited way.